Although a relatively recent addition to communication technology, time division multiple access (TDMA) has become an acceptable technique for use with transponders located in synchronous or quasi-synchronous earth orbit. Conventionally, a satelite transponder has an associated up-link frequency on which it receives from a plurality of earth stations. The transponder further has a down-link frequency on which it transmits. Time division techniques place a number of channels in a single frequency. To allow multiple stations to talk to each other through a single transponder on common up-link and down-link frequencies, proper timing is essential. To prevent garbling, the transmissions from each of the different stations must be received in time sequence at the transponder. In order to efficiently utilize the capacity of the transponder, proper timing is achieved through timing from a common marker or reference burst transmitted to each of the stations through the transponder. A periodic reference burst defines a frame, between adjacent reference bursts, which is further conceptually subdivided into a plurality of time slots. Each of the stations is allowed to transmit, in burst form, and each burst is aimed at a different slot. Thus, the frame includes the reference burst, and a plurality of slots for traffic. To make control of the network tractable, a further portion of the frame is provided for control purposes, i.e., to allow changing the slot allocations among the stations, etc.
TDMA systems can be further subdivided into single channel per burst systems and multiple channel per burst systems. In the former, time slots in the frame are usually of fixed duration and when a station needs a communication channel it must seize an unused time slot. In the latter type of system, each burst includes multiple channels and the time slots are dedicated to a particular station's bursts which may not be of fixed duration. Changing traffic among the stations is handled by changing burst (or slot) duration and, of necessity, burst position. Systems which are arranged to control changes in burst duration and position are disclosed in prior U.S. Pat. Nos. 3,644,678 (issued Feb. 22, 1972); 3,649,764 (issued Mar. 14, 1972); and 3,818,453 (issued June 18, 1974). The first two mentioned patents do not disclose multi-transponder systems. The latter patent, while disclosing how a particular station may transmit to, or receive from, multiple transponders and how burst duration and position may be varied does not disclose how the latter function can be achieved in a multi-transponder system in an orderly fashion. The present invention is directed to this end. A copending application, Ser. No. 779,874, filed on Mar. 21, 1977, relates to single channel per burst systems.
It is therefore an object of the present invention to provide for burst duration and location changes in a multiple transponder multiple channel per burst fully variable demand assignment system.